Asymptotic analysis of outwardly propagating spherical flames

Asymptotic analysis is conducted for outwardly propagating spherical flames with large activation energy. The spherical flame structure consists of the preheat zone, reaction zone, and equilibrium zone. Analytical solutions are separately obtained in these three zones and then asymp- totically matched. In the asymptotic analysis, we derive a correlation describing the spherical flame temperature and propagation speed changing with the flame radius. This cor- relation is compared with previous results derived in the limit of infinite value of activation energy. Based on this correla- tion, the properties of spherical flame propagation are inves- tigated and the effects of Lewis number on spherical flame propagation speed and extinction stretch rate are assessed. Moreover, the accuracy and performance of different mod- els used in the spherical flame method are examined. It is found that in order to get accurate laminar flame speed and Markstein length, non-linear models should be used.

CO_(2)-diluted CH_(4)-air premixed spherical flames with microwave-assisted spark ignition

The performance of microwave-assisted spark ignition(MAI)under exhaust gas recirculation conditions was explored with CO_(2)-diluted CH-air premixed spherical flames in a constant volume combustion chamber.The flame kernel radius at 5 ms after spark started was selected to evaluate the property of MAI for CO_(2)dilution ratio of 0-20%and equivalence ratio of 0.6-1.4 with 1 kHz microwave pulse repetition frequency under 0.2 MPa ambient pressure.The results showed that the addition of microwave induced some wrinkles on the flame surface and strongly deformed the flame.MAI expanded the limit of CO_(2)dilution ratio to 16%with an equivalence ratio of 0.75,in which case the spark only(SI)failed to ignite the mixture.With the CO_(2)dilution ratio increasing,the wrinkles induced by microwave pulses decreased apparently,and the enhancement value of MAI peaked at 4%CO_(2)dilution ratio.The effect of microwave was considered in two aspects,namely,reaction kinetics and thermal effect,which shows a“trade-off”as CO_(2)dilution ratio rose.With 8%volume of CO_(2)added,the flammable interval(equivalence ratio 0.6-1.2)of mixture in SI mode shrunk,and MAI can maintain a flammable interval consistency with the case that no CO_(2)was added.

Laminar Flame Instability of n-Hexane,n-Octane,and n-Decane in Spherical Expanding Flames

This paper focuses on the laminar flame instability of three high molecular weight n-alkanes,namely n-hexane,n-octane,and n-decane.The experiment was carried out in a constant volume combustion bomb to get the flame images.The critical radius under different conditions was extracted using the image processing program.Combined with the existing critical Peclet number theory,the dominant factors of flame instability under current conditions for three n-alkanes can be figured out.Moreover,the average cell size(equivalent cell radius,R_(cell))was extracted to provide quantitative analysis of the flame cellular structure,based on the method developed in this work.The theoretical R_(cell)were also calculated and compared with the experimental results to validate the proposed method.

Comparison of 2-acetylfuran,2-ethylfuran,and 2-methylfuran spherically expanding flame intrinsic instabilities

The intrinsic instability of three furan derivatives, namely, 2-acetylfuran(AF), 2-ethylfuran(EF), and 2-methylfuran(MF), is experimentally and theoretically investigated and compared. A deep-learning-based end-to-end system is used to segment the cells and measure the crack length on the cellular flame front of the spherically expanding flame to quantitively investigate the flame morphology. Moreover, the flame front radius profile is quantitively investigated to shed light on the correlation between disturbance amplitude and flame radius. With the use of the linear stability model, the theoretical critical flame radius and Peclet number are obtained and compared to their corresponding experimental results to characterize the outset of cellularization and evaluate the flame instability propensity. Our analysis shows that compared with AF and EF, MF is prone to cellularization at a smaller flame radius and has a faster cell proliferation rate and crack length growth rate before reaching the hydrodynamic saturation limit. Meanwhile, the average area of saturated cells tends to be similar for the three furan derivatives. Furthermore,AF premixed flame does not exhibit cellularization before the flame radius is less than 45 mm in low initial pressure.